The invention concerns a method for controlling the temperature of a refrigeration unit having a refrigerating compartment, a freezing compartment and a common evaporator of a cooling circuit, in which in a first mode (normal operation) the compressor of the cooling circuit is turned ON when cooling is required and turned OFF when cooling is not required, in dependence of the refrigerating compartment temperature, and a temperature control arrangement for a refrigeration unit with a refrigerating compartment having a refrigerating compartment temperature sensor, a freezing compartment, a common evaporator of a cooling circuit and a switch device, which in a first mode (normal operation) turns the compressor of the cooling circuit ON when cooling is required and OFF when cooling is not required, in dependence of the refrigerating compartment temperature.
Refrigeration units, in which the refrigerating compartment and the freezing compartment are cooled by a common evaporator, are normally dimensioned so that with normal operating conditions, for example with an ambient temperature of 25.degree. C. and a medium setting of the desired refrigerating compartment temperature of about 5.degree. C., a freezing compartment temperature of -18.degree. C. is obtained. Difficulties occur, when the ambient temperature decreases relative to the desired refrigerating compartment temperature set by the user. It is a fact that the cooling requirement of the refrigerating and freezing compartments decreases with decreasing ambient temperature; as, however, the cooling requirement of the refrigerating compartment decreases faster than that of the freezing compartment, it may happen that the cooling requirement of the refrigerating compartment is covered, whereas there is still a cooling requirement in the freezing compartment. As the cooling requirement of the refrigerating compartment is only small, the compressor is only turned on infrequently. This causes an inadmissible rise in the freezing compartment temperature. In the extreme case, when the ambient temperature is equal to or lower than the desired refrigerating compartment temperature, the compressor is not turned on at all, and the temperature of the freezing compartment will increase towards 5.degree. C. until a cooling requirement occurs in the refrigerating compartment again. This means that the freezing compartment temperature increases inadmissibly and that the freezing compartment temperature is dependent on the cooling requirement in the refrigerating compartment.
To solve the problem described, it is known (DE 196 48 399 A1) to provide a lamp or another heat source in the refrigerating compartment, which artificially increases the cooling requirement of the refrigerating compartment. This indeed leads to an earlier turning on of the compressor, but it also causes a higher energy consumption.
A solution involving control techniques is described in U.S. Pat. No. 5,524,444. Ex works an electronic table is prepared for each particular refrigerator unit, which table comprises ON-times in dependence of the ambient temperature for a certain desired refrigerating compartment temperature. These tables are calculated by the producer by means of tests, and comprise the ON-times of a compressor belonging to certain ambient temperatures. An electronic monitoring arrangement measures the ON-time of the compressor and calculates a medium value over three ON-times. This medium value is used as input value in the table and supplies information about the ambient temperature (calculated by means of tests) and the optimum ON-time of the compressor for maintaining the desired freezing compartment temperature. As the optimum ON-times for the freezing compartment and the refrigerating compartment are different, a compensation period is started to reduce the temperature in the freezing compartment.
From U.S. Pat. No. 5,535,597 it is known to forcedly turn on a compressor of a cooling circuit for predetermined times, when a temperature sensor is defective.
The task of the invention is to provide a method and a temperature control arrangement as described in the introduction ensuring in a simple way that the cooling requirements of both freezing compartment and refrigerating compartment are covered within a larger range of the ambient temperature.
With regard to the inventive method, this task is solved in that in a second mode, in spite of no cooling requirement in the refrigerating compartment, the compressor is forced ON for a predetermined operating time, when the OFF-time of the compressor has exceeded a threshold value, that the forced turning ON is repeated as often as the OFF-time exceeds the threshold value, and that a return to the first mode occurs as soon as the refrigerating compartment requires cooling.
With this method, the duration of the OFF-time is used for determining the cooling requirement in the freezing compartment, and the compressor is turned on accordingly once or several times for a predetermined operating period. Surprisingly, this measure permits the cooling requirement of the freezing compartment to be covered in a simple way, independently of the ambient temperature.
It is advantageous that in the second mode the compressor is turned OFF before reaching the end of the predetermined operating period, when the refrigeration compartment temperature goes below a safety value fixed to be somewhat over the freezing point. This ensures in a reliable way that the refrigerating compartment remains frost-free and that the articles stored in it are not damaged.
Further, it is recommended that the threshold value of the OFF-time is so high that the evaporator is defrosted. Thus, the evaporator is completely de-iced. This can be obtained in that the threshold value is found empirically and fixedly stored.
In a preferred embodiment the threshold value of the OFF-time is determined by the evaporator temperature rising to a value lying above the dew-point. Thus, it is ensured that the evaporator is completely defrosted, and, on the other hand, that the temperatures in the compartments do not increase unnecessarily.
With regard to the control arrangement, the task is solved according to the invention in that a monitoring arrangement measures the duration of the OFF-time, that a threshold value transmitter is provided, specifying a threshold value for the OFF-time, that a comparator compares the duration of the OFF-time with the threshold value during the OFF-time and, in case it is exceeded, emits an excess signal, that an ON-timer is provided, specifying an ON-time, and that in a second mode the switching device turns ON the compressor on the occurrence of an exceess signal and turns it OFF again at the end of the predetermined ON-time. This gives a relatively simple arrangement with which the method according to the invention can be carried through.
It is recommended that an overriding device is provided, which reduces the ON-time to a value, at which the refrigerating compartment sensor measures a temperature, which is slightly above the freezing point. Thus, the compressor can be turned OFF, when there is a risk of frost in the refrigerating compartment.
Further, it is advantageous that an evaporator temperature sensor and an overriding device are provided, which reduce the OFF-time to a value, at which the evaporator temperature sensor measures a temperature, which is slightly above the dew-point. This again provides a proper de-icing of the evaporator without causing an unwanted temperature increase in the refrigerating compartment and the freezing compartment.